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Effects on cognition and psychomotor performance
Not surprisingly, cannabis impairs cognitive and psychomotor performance. The effects are similar to those of alcohol and benzodiazepines and include slowing of reaction time, motor incoordination, specific defects in short-term memory, difficulty in concentration and particular impairment in complex tasks which require divided attention. The effects are dose-related but can be demonstrated after relatively small doses (5-10 mg THC in a joint), even in experienced cannabis users, and have been shown in many studies across a wide range of neurocognitive and psychomotor tests. These effects are additive with those of other central nervous system depressants.
Driving and piloting skills
These effects combine to affect skills related to driving a vehicle or flying an aeroplane. Numerous studies have shown that cannabis impairs road-driving performance and have linked cannabis use with increased incidence of road traffic accidents. In the UK, USA, Australia, New Zealand and many European countries, cannabis is the most common drug, apart from alcohol, to be detected in drivers involved in fatal accidents or stopped for impaired driving. A large proportion of such drivers have not taken alcohol or have concentrations below the legal limit. For example, in two studies from the UK Department of Transport
(Everest et al, 1989; Department of Environment, Transport and the Regions, 1998), no alcohol was detected post-mortem in 70% and 80%, respectively, in road traffic accident fatalities testing positive for cannabis. In Australia
(Road Safety Committee, 1995) only half of surviving drivers of vehicle collisions involving death or life-threatening injuries who tested positive for cannabis had also taken alcohol. In Norway, 56% of a sample of drug-impaired drivers negative for alcohol gave positive blood samples for THC
(Gjerde & Kinn, 1991). From the USA,
McBay (1986) had earlier found that 75% of a sample of drivers with cannabinoids in their blood were also intoxicated with alcohol. The
World Health Organization (1997, p. 15) concluded:
"There is sufficient consistency and coherence from experimental studies and studies of cannabinoid levels among accident victims...to conclude that there is an increased risk of motor vehicle accidents among persons who drive when intoxicated with cannabis.... The risk is magnified when cannabis is combined with intoxicating doses of alcohol"."
